All systems for manufacturing pulp, paper and paperboard include a series of operations and processes. Typically, wood is either digested chemically or ground up mechanically to form pulp. The chemical pulp must be washed, and both grades are often bleached. Pulping, bleaching and washing are generally carried out in the pulp mill; subsequent operations take place in the paper mill.
In the preparation of paper from pulp stock, water contributes in several ways. In addition to providing a medium through which shear forces may be transmitted to the fibers during beating, water acts as a vehicle of suspension in which the fibers, having been well dispersed, can be brought together to give a sheet having the desired formation.
Refining is one of the last steps to take place prior to dilution with process white water to form headbox furnish. The papermaker has the ability to control this important operation in order to optimize subsequent process and physical property parameters.
Refining consists of pumping the pulp slurry through a series of metal discs moving at a high speed controllable by the papermaker. During refining, the cellulose fibers are swollen, cut and macerated in order to develop fibrillation. This fibrillation increases the number of interfiber contacts during formation of the paper and bonding during subsequent pressing and drying operations. For example, a sheet that is formed from an unbeaten pulp has a low density and is rather soft and weak. If the same pulp is well-beaten, however, the resultant paper is much more dense, hard, and strong.
After refining, the pulp slurry is reduced in consistency by the addition of white water, prior to being pumped to the headbox. The concentration of solids in the headbox furnish is referred to as "consistency" and it typically ranges between 0.2 and 1%. In general, the lower the consistency the better the formation or homogeneity of appearance. From the headbox, the furnish is pumped onto a wire which, on a modern machine, can be moving at a speed of about 700 to 2000 m/min.
Continuous sheet forming and drying can be accomplished using three different types of equipment: the cylinder, Fourdrinier (i.e., single wire), and twin-wire machines. In the former, a wire-covered cylinder is mounted in a vat containing the refined fiber slurry. As the cylinder revolves, water drains inward through the screen, thus forming the paper web on the outside of the cylinder. The wet web is removed at the top of the cylinder, passes through press rolls for water removal, and is then passed over steam-heated, cylindrical drying drums. The Fourdrinier machine is more complex and basically consists of a long continuous synthetic fiber or wire screen (the "wire") which is supported by various means to facilitate drainage of water. The fiber slurry, which is introduced at one end of the machine through a headbox and slice, loses water as it progresses down the wire, thereby forming the web upon the screen. The web is then directed to the press and dryer sections as in the cylinder machine.
The twin-wire machine is the latest development and consists essentially of two opposing wires. Twin-wire formers have replaced the Fourdrinier wet-ends on many machines, particularly for lightweight sheets, e.g., tissue, towel, and newsprint. Twin-wire formers also are operated successfully on fine paper, corrugated media, and liner board grades. In twin-wire formers, the water is drained from the slurry by pressure rather than by a vacuum. The two wires, with the slurry between, are wrapped around cylinder or set of supporting bars or foils. The tension in the outer wires results in a pressure which is transmitted through the slurry to the supporting structure; the pressurized slurry drains through one or both of the wires.
Subsequent to stock preparation and dilution, the paper furnish is usually fed to the paper machine through one or more screens or other filtering devices to remove impurities. It then enters a flow spreader which provides a uniform flowing stream along the width of the paper machine. The flow spreader discharges the slurry into a headbox, where fiber agglomeration is prevented by agitation. Pressure is provided to cause the slurry to flow at the necessary velocity through the slice and onto the moving wire.
The wire is mounted over the breast roll at the intake end and at the couch roll at the discharge end. Between the two rolls, it is supported for the most part by table rolls, foils and suction boxes. A substantial vacuum is developed in the downstream nip between the table roll and the wire, and promotes water drainage from the slurry on the wire. As speeds increase, however, the suction can become too violent and deflect the wire, causing stock to be thrown into the air. A more controlled drainage action is accomplished by the use of foils. These are wing-shaped elements which support the wire and induce a vacuum at the downstream nip. Foil geometry can be varied to provide optimum conditions. After passing over the foils or table rolls, the wire and sheet pass over suction boxes, where more water is removed. Most machines also include a suction couch roll for further water removal.
In its most typical form, the formation of the paper web takes place in the first few feet on the screen of the papermaking machine. The stock issuing from the slice is a suspension of fibers in water, typically containing from 0.2 to 1.0% dry solids in a layer some 6-18 mm deep and up to several meters wide. It is deposited on, and drains through, an endless band of a woven synthetic fiber or metal fabric, called wire. At very low speeds, the force of gravity predominates in causing the drainage. At higher speeds, the action of gravity becomes negligible compared with the pumping action of the drainage elements (i.e., the table rolls or foils). A visible change occurs in the appearance of the stock as it proceeds down the wire when its concentration reaches about 2%. At this level, the surface ceases to appear mobile, loses its liquid sheen, and takes on a matte appearance. At this point in the process, the drainage elements are no longer effective for removing water because the web is formed. Next, consolidation begins, assisted by the action of the suction boxes. Some slight rearrangement of the fibers may still be achieved by the pressure of a light, permeable roller. Then the web will be taken from the roll for subsequent pressing, drying and finishing.
The sheet leaving the wet end has a consistency of 18-23%. Thus, it is possible to remove additional water mechanically without adversely affecting sheet properties. This is achieved in rotary presses, of which there may be one or several on a given paper machine. The press rolls may be solid or perforated and, often, suction is also applied through the interior of the rolls. The sheet is passed through the presses on continuous felts (one for each press), which act as conveyors and porous receptors of water. The water content of the sheet usually can be reduced by pressing to a consistency of about 30 to 40% without crushing.
Crushing, the direct flow of water in the sheet, occurs when too much pressure is applied to the wet sheet by the presses. Crushing can be avoided by applying pressure gradually, since less water is initially removed this way and the fibers are not so likely to be pushed apart. Also, crushing can be avoided by modifying the press rolls and felt construction to allow for increased water-removal rates. The sheet can stand higher and higher pressure as water is removed and the sheet becomes stronger. Graduated pressure is particularly important on heavy boards inasmuch as the danger of crushing increases for greater thicknesses of paper product. Pressing multicylinder boards while they are too wet may also lead to ply separation as well as crushing.
At a consistency of about 30 to 40%, additional water removal by mechanical means is not feasible and evaporative drying must be employed. This is a costly process and often is the production bottleneck of papermaking. The dryer section usually includes a series of steam-heated cylinders. Alternate sides of the wet paper are exposed to the hot surface as the sheet passes from cylinder to cylinder. In most cases, except for heavy board, the sheet is held closely against the surface of the dryers by a fabric having carefully controlled permeability to steam and air. Heat is transferred from the hot cylinder to the wet sheet, and water evaporates. The water vapor is removed by way of elaborate air systems. Most dryer sections are covered with hoods for collection and handling of the air, and heat recovery is practiced in cold climates. The final consistency of the dry sheet is usually between about 92-96 weight percent, depending upon the type of paper product being manufactured.
The efficiency of the drying sequence is dependent upon such factors as the amount of applied pressure which squeezes the wet web between the felts, the efficiency with which water condensed within the dryer cylinder is physically removed, the nature and conditions of the carrier felt, if any, and the ventilation of the pockets between dryers. During the drying sequence, the consistency of the product is increased from the entry level of generally about 30-40% up to that of the emerging dry paper product, i.e., 92-96%.
The energy requirements for removal of water depend upon the form of water which is present in the paper product. A major portion of free water, that which exists over and above what is required to saturate the fibers, can be removed on the wire by gravity or suction. Capillary water and an additional portion of the free water are removed by a pressing operation. The most tenaciously held water (i.e., that within the lumen and pores of the fiber wall) requires a significantly greater expenditure of energy for its removal, and this is generally accomplished utilizing thermal drying.
During the early stages of drying, the fibers are free to slide over one another, but as the free water is driven off, the fibers are drawn closer together and bonding begins to take place. Surface tension is primarily responsible for drawing together the fibers in this stage, but later, molecular attraction brings about the final bonding between fibers. No appreciable fiber-to-fiber bonding takes place until the consistency is raised above about 40 percent, but once the critical drying point is reached, shrinkage begins to take place and bonding begins.
In summary, the three steps which are necessary to form a final paper product from wood pulp all relate to the removal of water from the fiber or web. These include:
(1) depositing pulp upon a screen (or "wire") to form a web of paper fiber. This step, known in its initial stage as formation, is usually accomplished by passing an aqueous dispersion of a low concentration of pulp (e.g., 0.2% to 1%) over the screen. This screen, assisted in certain situation by vacuum or suction, increases the consistency of the web to approximately 18 to 23 percent.
(2) compressing or squeezing the web in a "press section" to further remove water. This is usually accomplished by felt presses, a series of rollers each having a felted band for contact with the web. These presses remove additional free water and some capillary water, thus resulting in an increase in consistency of the web to a range of about 30 to 40 weight percent.
(3) drying the web utilizing steam-heated equipment in a "dryer section." Here, the remaining water content of the web is reduced to that desired for the final specific product, the consistency of which typically ranges between about 92 to 96 weight percent.
As mentioned above, the greatest energy use occurs during the drying of the paper product. For example, in the manufacture of thicker grades of paper product, such as board, in one case 88.6% of paper mill steam usage was reported to be at the drying cylinders.
Drying is a relatively expensive process, and the cost of drying is always a major part of the processing cost of the final paper, thus any significant savings in energy in the drying stage would directly result in significant cost savings.